The present invention relates to a method and apparatus for performing confinement. It relates more precisely to a method of confining pollution generated in the top volume and/or in the bottom volume of an enclosure filled with a fluid, and to apparatus associated with said method. Said method is industrial and it is original in that it is based on the natural phenomenon of thermal stratification. When it is implemented, it can surprisingly guarantee effective confinement in various contexts, and in particular in the most unfavorable context in which a hot pollution source is disposed at the bottom of an enclosure whose top portion is to be protected from said pollution source. The Applicant has developed the method and apparatus of the invention specially for the context of vitrifying fission products in the nuclear industry, with a view to protecting equipment of the hoist type disposed at the top of the vitrification cell from pollution given off by the melting pot and by the calciner. However, said method and apparatus of the invention, which method and apparatus are described in detail below, are in no way limited to this context.
In the prior art, the phenomenon of fluid stratification is well known.
In Liquids
Gravity stratification is commonly observed in non-miscible liquids of different densities. It is very stable and requires high energy to cause the two phases to be mixed (emulsion). In the absence of an emulsion, the relatively small area of the interface per unit volume constitutes an effective barrier against the transfer of a solute or of particles in suspension from one phase to the other.
In Gases
A similar phenomenon can be observed, if the two phases are gases of different densities situated in an enclosure of large volume in which the ambient environment is disturbed little.
In this case, the stratification phenomenon is much less stable and the interface is less clearly defined than for the interface between liquid phases. The interface is replaced with a xe2x80x9cmixing zonexe2x80x9d due to Brownian motion and turbulent diffusion in which the mean concentration of one phase in the other varies continuously with a steep gradient when going up the vertical axis.
With reference more particularly to thermal stratification, it is known that differences in density exist in the same fluid because of differences in temperature; the fluid can then behave as two distinct phases, namely a cold phase and a hot phase. The two phases are not very miscible if the volumes involved are large, and they can thus present the same stratification phenomena as fluids of distinct compositions. This natural phenomenon of thermal stratification explains the following:
the ocean currents;
the meteorological phenomenon of temperature inversion and its effects on atmospheric pollution; and
the temperature profile (as a function of depth) of water in a mountain lake.
In the context of the present invention, it has been observed, surprisingly, that it is possible to control said natural phenomenon of thermal stratification so as to use it to create genuine confinement barriers artificially in horizontal planes both in liquids and in gases. As a person skilled in the art can easily understand, it is far from obvious that such control can be achieved. Indeed, it would seem unlikely to be possible for it to be achieved, in particular in a gaseous atmosphere which is very sensitive to convection currents and to turbulence. Preconceived opinion was therefore strongly unfavorable to basing an industrial method of confinement on the natural phenomenon of thermal stratification.
Known methods of confinement, implemented currently and while the invention was being developed, in particular to protect equipment from a polluting atmosphere charged with particles, are of the following types:
close protection with the use of a covering;
protection by curtains of air;
protection under laminar flow; and
protection based on thermophoresis.
After becoming acquainted with the present invention, the person skilled in the art can appreciate the advantages that the invention offers in various contexts over the above-mentioned prior art techniques. At this stage of the description, it is worth emphasizing that the method of the invention is effective, and that the accessory equipment required to implement it is compact.
It is proposed to describe below the two aspects of the invention, namely the method and the apparatus, in a general manner in a first part, and in a more detailed manner in a second part, with reference to a particular implementation.
The invention thus provides firstly a method of confining pollution generated in the top volume and/or in the bottom volume of an enclosure filled with a fluid, i.e. either a gas, which is in general air, or a liquid, which is in general water, the method confining the pollution by thermal stratification: the mean temperature of said top volume is maintained higher than the mean temperature of said bottom volume by an amount that is sufficient to ensure that said two volumes are separated by a turbulent intermediate zone of narrow width, referred to as the xe2x80x9cmixing zonexe2x80x9d, within which a steep temperature gradient is maintained; said intermediate zone constituting a virtual confinement barrier acting as a virtual partition in a horizontal plane.
Said method consists in creating artificially a confinement barrier between the top volume and the bottom volume of the enclosure by maintaining a sufficiently large temperature difference between said top and bottom volumes (the temperature of said top volume being obviously maintained higher than the temperature of said bottom volume). Said temperature difference must guarantee a sufficiently large density difference between the hot fluid in the top volume and the cold fluid in the bottom volume. Said density difference must be such that the effects of vertical forces, directed downwards for the cold fluid and upwards for the hot fluid (said forces, which are due Archimedes"" thrust, being applied to the various volumes of the two phases, and tending to separate them by stratification) prevail over the effects of inertia forces due to the rate at which said volumes penetrate into the mixing zone. The inertia forces are due to the random speeds in the turbulence of the ambient environment, and they are responsible for diffusion mixing and for heat exchange between phases. Thus, by means of the invention, the enclosure is artificially subdivided into two distinct enclosures.
As specified above, the method of the invention may be implemented in an enclosure filled with gas (the enclosure is then more readily referred to as a xe2x80x9ccellxe2x80x9d or a xe2x80x9croomxe2x80x9d), or in an enclosure filled with liquid (such as a xe2x80x9cpoolxe2x80x9d). In general, only one fluid, i.e. either a gas or a liquid, is involved. When the fluid is a gas it consists, in general, of air; when it is a liquid, it consists, in general, of water. Other gases, such as nitrogen, for example, and indeed other liquids are in no way excluded from the ambit of the invention. Similarly, the invention does not exclude the possibility of two different types of gas or of two different types of liquid being present in the enclosure. However, in such an event, the densities of the two fluids involved must be compatible with implementation of the method.
The method of the invention confines the two volumes of the enclosure, namely the top volume and the bottom volume, relative to each other, and any pollution being generated in one of said volumes is kept out of the other volume, or if pollution is being generated in both of said volumes, then each of said volumes is protected from the pollution being generated in the other volume.
The pollution may be of various types. Its source may, for example, consist of a mechanical source of dust, in particular radioactive dust (such as a sawing station, a shearing station, or a welding station, in general located at the bottom of an enclosure; such a station may equally well be positioned at the top of an enclosure, in particular when work is to be done on the top of equipment such as a rocket), or else the source of the pollution may consist of (optionally hot) feedstock emitting vapor charged with particles (such as the melting pot and the calciner that are disposed at the bottom of a cell for vitrifying fission products).
Such a pollution source may be cold or hot, and it may be disposed at the top of and/or at the bottom of the enclosure. All cases are possible, the least favorable case being when a hot pollution source is disposed at the bottom of the enclosure. The pollution given off by such a source tends to pollute the top zone naturally by convection. The method of the invention can be implemented effectively in the various other cases, and it is also effective in this difficult context which is particularly difficult when operating in a gaseous atmosphere.
Suitable means are used to maintain the top and bottom volumes of the enclosure at temperatures such that thermal stratification is established in stable manner. In an advantageous variant of the method of the invention, the top volume and the bottom volume are swept independently with fluid injected at respective suitable temperatures; the fluid injected into the top volume being extracted therefrom immediately above the top interface of the mixing zone (confinement barrier), and the fluid injected into the bottom volume being extracted therefrom immediately below the bottom interface of said mixing zone; said fluids being either identical or different, and being injected into the respective volumes under conditions that minimize the vertical component of the speed of the turbulence that is generated.
This advantageous variant may be implemented either:
with a gas (or even two gases, as mentioned above), such as air, that is blown hot into the top volume and that is blown cold into the bottom volume; or
with a liquid (or even two liquids, as mentioned above), such as water, that is injected hot into the top volume and that is injected cold into the bottom volume.
In any event, to obtain optimum effectiveness, which means that the generated confinement barrier must be stabilized, it is necessary to ensure that the hot and cold fluids are injected xe2x80x9csmoothlyxe2x80x9d. Naturally, said fluids are advantageously also extracted smoothly, but the disturbances generated on extraction are less harmful. The disturbances generated by the injection thus need to be minimized. For this purpose, the vertical component of the speed of the turbulence generated is minimized. Advantageously, and for the same purpose, it is sought to ensure that speeds are uniformly distributed at the outlet sections of the injection means, and said injection means are disposed as far as possible from the mixing zone constituting the confinement barrier. It can be understood that, conversely, the means for extracting the injected fluids are situated as close as possible to said mixing zone.
In an advantageous variant implementation of the invention, the hot fluid sweeping the top volume of the enclosure is recycled, at least in part.
In the method of the invention, a confinement barrier is thus created in a horizontal plane by maintaining a sufficiently large temperature difference between the bottom portion (cold zone) and the top portion (hot zone) of the enclosure. The temperature difference may be generated by any means. As mentioned above, it advantageously results from said bottom and top portions being swept with fluid(s) at suitable temperatures.
The present invention also provides apparatus suitable for implementing the above-described method of confinement. In characteristic manner, said apparatus for confining pollution generated in the top volume and/or in the bottom volume of an enclosure filled with a fluid, i.e. either a gas, which is in general air, or a liquid, which is general water, comprises:
temperature-maintaining means for maintaining the mean temperature of said top volume greater than the mean temperature of said bottom volume thereby creating a virtual confinement barrier in a horizontal plane between said top volume and said bottom volume, said virtual confinement barrier being constituted by a narrow, turbulent intermediate zone referred to as the xe2x80x9cmixing zonexe2x80x9d, within which a steep temperature gradient is maintained;
and advantageously means for thermally-insulating at least some of the walls of said top volume.
The person skilled in the art understands that several variants are possible for said means for maintaining the top and bottom volumes of the enclosure at the suitable temperatures, and that, in any event, it is advantageous to insulate said top volume thermally. It is advantageous firstly to reduce heat exchange and secondly to avoid too large a temperature difference between said walls of said top volume and the ambient atmosphere. Such a large temperature difference would give rise to convection currents and to harmful turbulence.
In the context of the advantageous variant of the method of invention in which the top volume and the bottom volume are swept with fluid maintained at respective suitable temperatures, it is recommended for the temperature-maintaining means for maintaining the suitable temperatures in each of said volumes to comprise fluid delivery apparatus and fluid extraction apparatus suitably disposed in each of said volumes, and suitable for ensuring that each of said volumes is swept. Said delivery apparatus is of shape and size optimized to reduce the vertical speed component of the turbulence generated. Naturally, said delivery apparatus is connected upstream to means suitable for feeding in the fluid at the desired temperature, and said extraction apparatus is connected downstream to suitable means for sucking out said fluid once it has swept the volume in question (top volume or bottom volume).
It is recommended for the extraction apparatus for extracting the hot fluid from the top volume (more precisely from the bottom of said top volume), and for the extraction apparatus for extracting the cold fluid from the bottom volume (more precisely from the top of said bottom volume) to consist of narrow slots distributed uniformly on respective common levels and facing one another along the entire length of two opposite vertical walls of the enclosure. Naturally, care is taken to ensure that the vertical walls in which the slots are provided are not structurally weakened thereby. Each slot is actually subdivided into a plurality of slot elements. When the enclosure is a rectangular block, the person skilled in the art can understand that said extraction slots are advantageously disposed along the longitudinal (horizontal) axis of said enclosure.
It is recommended for said delivery apparatus for delivering the hot fluid into the top volume (more precisely into the top of said top volume) and/or for said delivery apparatus for delivering the cold fluid into the bottom volume (more precisely into the bottom of said bottom volume) to consist of
a) a respective horizontal surface ensuring that said fluid is distributed continuously; or
b) at least two narrow slots distributed uniformly and parallel over the entire length of a horizontal wall (floor and/or ceiling) of the enclosure; or
c) two respective series of slots of narrow width and of small height distributed uniformly in staggered manner over the entire length of two opposite vertical walls of the enclosure; said slots starting at or in the vicinities of the horizontal wall(s) (ceiling and/or floor) in contact with said vertical walls.
In variant a), the floor (or false floor) and/or the ceiling (or false ceiling) of the enclosure is perforated over at least part of its surface, thereby constituting a wall for diffusing the injected fluid.
In variant b), longitudinal slots (in general at least two such slots) are provided to ensure that sweeping is effective. Said slots may be organized in the same manner in a false ceiling or in a false floor.
In variant c), two series of slots are provided in the bottom and/or the top of vertical walls of the enclosure. These slots are distributed in staggered manner (the two series are advantageously uniformly offset) so as to minimize the turbulence generated on injection. Advantageously, the slots that are provided in the bottom volume of the enclosure are not at floor (or false floor) level, but rather they are slightly above floor (or false floor) level. This avoids stirring up any dust that has settled on said floor (or false floor).
The apparatus of the invention may use the same type or different types of fluid delivery apparatus in the top and bottom volumes of the enclosure. In an advantageous variant, in the volume(s) in which the pollution is generated, namely the top volume and/or the bottom volume, the fluid delivery apparatus is of above type c). This type of apparatus is particularly optimized for minimizing the vertical speed component of the turbulence generated on injection.
Advantageously, this type of apparatus provided in the polluted top or bottom volume is associated with apparatus of above type b) in the corresponding non-polluted bottom or top volume (assuming that the pollution is generated in one of said volumes only).
The means of the apparatus of the invention that are provided for ensuring that the top volume is swept with a hot fluid advantageously include means for recycling said hot fluid at least in part.
Similarly, the means of the apparatus of the invention that are provided for ensuring that the bottom volume is swept with a cold fluid are advantageously fed with a fluid at ambient temperature or with fluid cooled upstream, advantageously by means of a heat pump that uses the heat energy taken from said fluid to raise the temperature of the fluid fed to the hot fluid delivery apparatus.
In the context of the invention, it is thus possible by various means (thermally insulting, recycling, providing heat exchangers) to optimize the energy efficiency of the method.
In a variant embodiment, suitable in particular for confining pollution given off by a possibly hot pollution source disposed at the bottom of an enclosure filled with a gas, which is in general air, said apparatus of the invention comprises:
in its bottom volume, delivery apparatus of above type c) for delivering a cold gas, in general air;
in its top volume, delivery apparatus of above type b) for delivering a hot gas, in general air; and
in each of its top and bottom volumes, extraction apparatus of the slot type, as described above, for extracting the injected gas, in general air.
This variant is illustrated below.